THE MEMBRANE-PROXIMAL INTRACYTOPLASMIC TYROSINE RESIDUE OF HIV-1 ENVELOPE GLYCOPROTEIN IS CRITICAL FOR BASOLATERAL TARGETING OF VIRAL BUDDING IN MDCK CELLS

Budding of retroviruses from polarized epithelial Madin-Darby canine k idney cells (MDCK) takes place specifically at the basolateral membran e surface. This sorting event is suspected to require a specific signa l harbored by the viral envelope glycoprotein and it was previously sh own that, as for most basolateral proteins, the intracytoplasmic domai n plays a crucial role in this targeting phenomenon. It is well known that tyrosine-based motifs are a central element in basolateral target ing signals. In the present study, site-directed mutagenesis was used to generate conservative or non-conservative substitutions of each fou r intracytoplasmic tyrosines of the human immunodeficiency virus (HIV- 1) envelope glycoprotein. This approach revealed that the membrane-pro ximal tyrosine is essential to ensure both the basolateral localizatio n of envelope glycoprotein and the basolateral targeting of HIV-1 viri ons. Substitutions of the membrane-proximal tyrosine did not appear to affect incorporation of envelope glycoprotein into the virions, as as sayed by virion infectivity and protein content, nor its capability to ensure its role in viral infection, as determined by viral multiplica tion kinetics. Altogether, these results indicate that the intracytopl asmic domain of the HIV-1 envelope glycoprotein harbors a unique, tyro sine-based, basolateral targeting signal. Such a tyrosine-based target ing signal may play a fundamental role in HIV transmission and pathoge nesis.